Decreased clinical response to infliximab in ankylosing spondylitis is correlated with anti-infliximab formation.

Objectives: Correlation of serum trough infliximab levels and antibodies to infliximab (anti-infliximab) with clinical response in ankylosing spondylitis.
Methods: In accordance with the international ASsessment in Ankylosing Spondylitis (ASAS) consensus statement, patients were treated with infliximab (5 mg/kg) every 6 weeks after a starting regimen. Preinfusion sera were collected at baseline, 24 and 54 weeks. At every visit, the 20% improvement response (ASAS-20) was assessed and laboratory tests performed.
Results: 24 of the 38 (63%) patients fulfilled ASAS-20 response criteria after 24 weeks of treatment and 21 (53%) after 54 weeks. After 54 weeks, 11 (29%) patients showed undetectable serum trough infliximab levels and detectable anti-infliximab; six of these patients developed an infusion reaction. Anti-infliximab was found significantly more often (p = 0.04) in ASAS-20 non-responders compared with responders at week 54. Serum trough infliximab levels were significantly (p<0.0001) lower in patients with (mean 0.02 mg/l) than in those without (12.7 mg/l) anti-infliximab.
Conclusions: In ankylosing spondylitis, high levels of serum trough infliximab correlated with a good clinical response. Detection of anti-infliximab within 54 weeks is associated with undetectable serum trough infliximab levels, reduced response to treatment and increased risk of developing an infusion reaction.